1. Field of the Invention
This invention relates to a high frequency energy apparatus, more in particular to an improvement in a control system for a high frequency energy apparatus such as a microwave oven for automatically heating an object to a proper temperature.
2. Description of the Prior Art
In conventional high frequency energy apparatuses for heating an object contained in a heating chamber by high frequency energy which is supplied to the heating chamber, the heating of the object is controlled by regulating the heating time with a timer or the like at the discretion of the user. A proper heating time, however, depends on such factors as the quantity, quality (for example, water content or component substances), initial temperature, and the shape of the object. The setting of the heating time therefore requires considerable skill, often causing overheated or underheated conditions of the object due to an improper setting of the heating time.
Various attempts have been made to overcome these disadvantages in automatically controlling the heating of the object by detecting the temperature of the object per se. One of them utilizes the fact that the temperature of the air in the heating chamber increases with the increase in temperature of the object on the pan in the heating chamber accordingly as it is heated by high frequency energy. In this case, the high frequency energy apparatus is typically provided with an exhaust hole through which the air in the heating chamber is exhausted from the chamber and an exhaust pathway through which the air is expelled out of the high frequency energy apparatus. A temperature-sensitive element is disposed in the vicinity of the exhaust hole or about halfway in this exhaust pathway to detect the temperature of the air exhausted from the heating chamber. In this way, the heating of the object may be controlled, by detecting the relative temperature of the object. As described in detail later, however, the gradient of the characteristic curve of the temperature of the object versus that of the exhaust air varies with the quantity or mass of the object to be heated. The result is that an object, which may be heated to the desired temperature in a certain quantity or mass, may fail to reach or may be heated above the desired temperature in another quantity or mass. This leads to a great variation in the final temperature, making proper heating very difficult. Further, it is not only the quantity or mass of the object but also the quality thereof that causes an overheated or underheated condition thereof.